The present invention relates to a method for forming an electrostatic latent image, and more particularly, to an improved method for forming an electrostatic latent image in electrophotography which can provide a high quality recording having excellent uniformity in the case of a recording containing a half-tone area.
Heretofore, an electrophotographic process has been known, in while an electrostatic recording medium such as an electrostatic recording paper sheet or a film of dielectric is urged against a photoconductive layer disposed on an electrode by means of a back surface electrode, the photoconductive layer is irradiated with and exposed to an optical image and also a voltage having a predetermined polarity is applied to said electrostatic recording medium, whereby an electrostatic latent image is formed on the electrostatic recording medium. Such a process is disclosed, for example, in U.S. Pat. No. 3,502,408. Generally, this type of electrophotographic process produces a reversed image after development with respect to the irradiated optical image. That is, an area corresponding to a bright area in the optical image is dark in the developed image, whereas another area corresponding to a dark area in the optical image is a bright area in the developed image. Although this electrophotographic process is suitable for the purpose of recording an image containing a half-tone area due to the fact that an electric charge corresponding in amount to the amount of exposure is transferred onto an electrostatic recording paper web, it has the short coming that gooseflesh nonuniformity occurs in half-tone area and the quality of picture is poor. Also the term "gooseflesh" non-uniformity as used herein means at those areas of a recording paper sheet which must properly have a uniform tone at a white or intermediate level, do not have a uniform tone, but a spotted pattern on the order of millimeters throughout the areas at a denser tone than the proper tone of those areas. Since this nonuniformity is marked when an electrostatic recording paper web isused as an electrostatic recording medium and it varies depending upon the varieties of photoconductive layers, the kinds of electrostatic recording paper webs, and the like, it is believed that the nonuniformity is caused by micro-fine fluctuations in the width of the gap clearance formed between the photoconductive layer and the electrostatic recording paper web due to micro-fine unevenness of of paper web and by the surface condition and thickness of the photoconductive layer and dielectric layer of the electrostatic recording paper web.
However, the mechanism of generation of the nonuniformity has not been known in detail because the width of the gap clearance as well as the electrostatic capacity of the photoconductive layer and the electrostatic recording paper web could not be measured precisely under practical recording conditions and hence it has been very difficult to improve the gooseflesh nonuniformity.
As one approach for reducing the nonuniformity of recording an electrophotographic process was proposed that would utilize an operating region having the maximum amount of electric charge transfer. More particularly a novel electrophotographic process was proposed in which a voltage that is 10 V or more higher in absolute value than the threshold value necessary for depositing electric charge on an area of a recording paper web corresponding to the minimum exposure area, is applied across a photoconductive layer to make electric charge deposit even onto a dark area of an optical image where originally electric charge would not deposit, thus forming a so-called excessive electrostatic latent image having many fog areas, upon development the latent image is developed while applying an inverse bias voltage for suppressing deposition of a charged toner, whereby the nonuniformity of recording is reduced. However, even with such a proposed process, the nonuniformity was eliminated perfectly, and hence it has been impossible to obtain a high quality recording having good uniformity such as the conventional optical photograph.